In recent years, with the advent of a ubiquitous network society, there have been increasing needs for using radio waves and a wireless personal area network (WPAN) for achieving a home wireless broadband network (wireless personal area network) or a millimeter-wave wireless system, such as a millimeter-wave radar, for supporting stable and safe operation has started to be used. In addition, measures for a 100-GHz ultra-wide band wireless system have been actively taken.
However, in the evaluation of the second-order harmonics of a wireless system with a bandwidth of 60 GHz to 70 GHz or the evaluation of wireless signals in a frequency band greater than 100 GHz, as the frequency increases, the noise level of a measuring device and the conversion loss of a mixer increase and frequency accuracy is reduced. Therefore, a technique for measuring wireless signals in a frequency band greater than 100 GHz with high sensitivity and high accuracy has not been established. In addition, it is difficult for the measuring technique according to the related art to separate harmonics of a local oscillating signal from the measurement result and to accurately measure, for example, unnecessary radiation.
In order to solve these technical problems and measure wireless signals in a frequency band greater than 100 GHz with high sensitivity and high accuracy, it is necessary to develop a narrow bandpass filter technique such as a millimeter waveband filter technique for suppressing an image response and a high-order harmonic response. In particular, a filter technique which can be applied to a variable frequency (tunable) type is preferable.
The inventors have proposed a millimeter waveband filter in which a Fabry-Perot resonator used in the optical field is applied to millimeter waves and which selectively transmits desired frequency components of millimeter waves using the resonance between a pair of radio wave half mirrors that are provided in a transmission line of a waveguide structure, which transmits the electromagnetic waves in a TE10 mode (single mode), so as to be opposite to each other (Patent Document
Patent Document 1 discloses a technique in which a transmission line that transmits electromagnetic waves in a desired frequency band in the TE10 mode is formed by a first waveguide and a second waveguide into which one end of the first waveguide is inserted with a slight gap therebetween and is fixed such that the radio wave half mirrors faces each other at the leading end of the first waveguide and in the second waveguide, and one of the waveguides is moved in the longitudinal direction relative to the other waveguide such that the gap between the radio wave half mirrors is changed.
According to the millimeter waveband filter with the above-mentioned structure, characteristics do not deteriorate due to wave front conversion and it is possible to improve flexibility in the design of the radio wave half mirrors. In addition, loss caused by spatial radiation is less and the gap between the pair of radio wave half mirrors can be changed to change the resonance frequency of the filter.
However, when the millimeter waveband filter with the above-mentioned structure is actually manufactured, it is necessary to provide a gap which enables the waveguides to move in the longitudinal direction relative to each other between the outer circumferential wall of the first inner waveguide and the inner circumferential wall of the second outer waveguide. The gap is continuous with the space of the resonator formed between the pair of radio wave half mirrors and electromagnetic waves which reciprocate between the radio wave half mirrors leak to the outside through the gap. As a result, the characteristics of the filter deteriorate.
Therefore, it is necessary to minimize the gap. For example, in the case of a waveguide including a transmission line with a size of about 2 mm×1 mm, the allowed gap is equal to or less than several tens of micrometers (for example, 20 μm to 30 μm), which are dimensions that can be observed only by a microscope. However, as in the millimeter waveband filter having the above-mentioned structure, in the structure in which the leading end of the first waveguide is inserted into the second transmission line, it is difficult to observe the gap from the outside and to check a variation in the gap. As a result, it is very difficult to position the waveguides.
As a technique for solving the above-mentioned problems, the inventors have proposed the following technique in Patent Document 2. In the technique, a second outer waveguide includes a first transmission line forming body and a second transmission line forming body. In the first transmission line forming body, a rectangular hole forming a first transmission line with a size which is capable of accommodating one end of a first inner waveguide is formed in a plate portion with a constant thickness to a thickness direction so as to pass through the plate portion. In the second transmission line forming body, a rectangular hole forming a second transmission line having the same size as the first waveguide is formed in a plate portion with a constant thickness in the thickness direction so as to pass through the plate portion. The plate portions of the first transmission line forming body and the second transmission line forming body can be connected to or separated from each other, with the rectangular holes overlapping each other so as to be concentrically continuous.
When this technique is used, it is possible to observe the gap between the outer circumference of the first inner waveguide and the rectangular hole forming the first transmission line in the second outer waveguide from the first transmission line forming body and to accurately position the first and second waveguides. After the positioning process, when the second transmission line forming body is connected to the first transmission line forming body at a predetermined position, the second transmission line is not inclined with respect to the first transmission line and it is possible to accurately position three transmission lines including the transmission line of the first waveguide. Therefore, it is possible to maintain high filter characteristics.